Brown dwarfs surprise
Well, it seems they form differently, for a start; According to Allard, brown dwarfs, like stars, form from the core collapse of a gas cloud, whilst planets form from the bottom up in core accretion. Planets also have a much richer atmospheric composition, with much larger amounts of heavy elements and several convection layers. Brown dwarfs rotate rapidly and according to Allard's computer simulations, they have thick cloud cover swathing their atmospheres, full of the likes of ammonia and silicate dust. Breaks can occur in the cloud cover, resulting in variability in their brightness, not that they are very bright in the first place - their relatively low temperature (as 'cool' as 300 degrees Celsius) means they are dim and only visible at infrared wavelengths. Most astonishingly of all, they have magnetic fields likes stars, and in some cases they can be incredibly powerful: one brown dwarf, TVLM 513-46546, has been observed to flare at radio wavelengths with energies 1,000 times greater than the Sun. Who'd have thought these 'failed stars' could pack such a punch?
Image: An artist's impression of a low mass dwarf, courtesy NASA/ESA/G Bacon (STScI).